Disinfection byproduct formation in swimming pools

Abstract

The fate of organic nitrogen and carbon introduced into a swimming pool by pool users has been studied using a 2.2m3 model pool. The study made use of a body fluid analogue, containing the primary endogenous organic amino compounds, and a soiling analogue represented by humic acid. The system was used to examine the effect of organic loading and organic carbon source (i.e. amino or humic acid) on the levels and speciation of the key chlorinated disinfection byproducts (DBPs) of trihalomethanes (THMs) and chloramines under operating conditions representative of those employed on a full-scale pool.
Results revealed organic carbon (OC), chloramines and THMs to all attain steady state levels after 200-500 hours of operation, reflecting mineralisation of the dosed OC. Steady state levels of OC were roughly linearly dependent on dose rate over the range of operational conditions investigated and, as with the chloramine levels recorded, were in reasonable agreement with those reported for full-scale pools. THM levels recorded were somewhat lower than those found in real pools, and were dependent on both on pH carbon source: the THM formation propensity for the soling analogue was around eight times than of the body fluid analogue. Of the assayed byproducts, only nitrate was found to accumulate, accounting for 4-28% of the dosed amino nitrogen. Contrary to previous postulations based on the application of Henry's Law, only insignificant amounts of the volatile byproducts were found to be lost to the atmosphere.
A mathematical model based on a consideration of equilibrium thermodynamics was shown to accurately predict the formation of chloramines and decay of chlorine for ex-situ closed-bottle experiments. Extension of the model to the pilot pool is problematic, however, since more empirical data from the chlorination of individual BFA constituents is required for this.